1. Field of the Invention
The present invention relates to an image forming apparatus wherein a developing device develops an electrostatic latent image formed on an image bearing member with toner to form a toner image.
2. Discussion of the Background
An image forming apparatus such as a copying machine, a printer, a facsimile, or a multifunctional image forming apparatus having at least two functions out of copying, printing, and faxing is known. Such an image forming apparatus forms toner images on a transfer material, for example a transfer sheet, by the following process steps: electrostatically forming a latent image of an image on an image bearing member; developing the latent image with toner by a developing device; and transferring the toner image to a transfer material by a transferring device.
When a toner replenishing device that contains powdered toner and supplies air to the toner to replenish a developing device with the toner is provided in the above-described image forming apparatus, the toner replenishing device allows the toner to be replenished to the developing device from a toner container provided at a distance from the developing device, and a space between the developing device and the toner container can be effectively used in the image forming apparatus. As a result, elements of the image forming apparatus can be designed with numerous layouts, and operability of the image forming apparatus can be enhanced.
Referring further to the above-described toner replenishing device, because fluidity of the toner is increased by supplying air to the toner, the toner is smoothly and effectively conveyed and replenished to the developing device.
In order to surely increase the fluidity of the toner, it is necessary to add a large quantity of external additives to the toner. In this case, the toner including the external additive can be smoothly and effectively conveyed and replenished to the developing device while supplying air to the toner.
However, when a large quantity of external additives are added to the toner as described above, a film of the additives added to the toner, a fine powdered toner, a paper powder of a transfer sheet, etc. is likely to adhere to the surface of the image bearing member.
Specifically, the image forming apparatus is configured such that a cleaning device removes toner and paper powder that remain on the surface of the image bearing member after a toner image is transferred to a transfer material. In this case, additives separated from the toner on the image bearing member are pressed by the cleaning device and adhere to the surface of the image bearing member. Then, finely powdered toner, paper powder, etc. stick to the pressed additives on the image bearing member. As a result, a film made of additives, finely powdered toner, paper powder, etc. (hereinafter may be simply referred to as a film of additives) adheres to the surface of the image bearing member.
If the occurrence of the aforementioned adhesion of the film of additives to the image bearing member becomes frequent, the image quality of the toner image formed on the surface of the image bearing member inevitably deteriorates.
When the above-described image forming apparatus is configured such that a toner returning device returns the toner removed by the cleaning device to the developing device for reuse in subsequent development, a toner consumption amount can be reduced, and resources can be effectively utilized.
When a mechanism for reusing toner (hereinafter referred to as a toner recycling mechanism) is employed, toner is circulated between the developing device and the cleaning device, and the external additive added to the toner typically becomes embedded in toner particles during the circulation. In this condition, the fluidity of the toner, owing to the external additive, becomes substantially lowered. For this reason, when the toner recycling mechanism is employed in the image forming apparatus, it is necessary to prevent the lowering of the fluidity of the toner due to external additive embedded in toner particles by adding a large quantity of external additives greater than the number of toner particles to the toner.
On the other hand, when a large quantity of external additives greater than the number of toner particles are added to the toner, a film made of additives added to the toner, finely powdered toner, paper powder, etc. is likely to adhere to the surface of the image bearing member as described earlier.
Referring further to the above-described image forming apparatus, a charging device charges the image bearing member, and an exposing device exposes a charged surface of the image bearing member to form an electrostatic latent image on the image bearing member.
When the charging device is configured to be applied with a voltage wherein an alternating current voltage is superimposed on a direct current voltage to charge the image bearing member, the image bearing member can be uniformly charged without a charging unevenness, even though an environmental condition (e.g., humidity, temperature, etc.) changes. As a result, high quality toner images can be formed.
However, as a result of a recent study, it has been found that the above-described film of additives is likely to adhere to the surface of the image bearing member and the wearing of the surface of the image bearing member typically advances, when a voltage wherein an alternating current voltage is superimposed on a direct current voltage is applied to the charging device, and when the toner to which a large quantity of external additives are added is used in the image forming apparatus.
The present invention has been made in view of the above-discussed and other problems, and an object of the present invention is to address these problems.
Another object of the present invention is to provide a novel image forming apparatus including a toner replenishing device that contains powdered toner and supplies air to the toner to replenish a developing device with the toner, that can prevent adhesion of a film of additives to a surface of an image bearing member and can form a high quality toner image even when a large quantity of external additives are added to the toner.
Another object of the present invention is to provide a novel image forming apparatus employing a toner recycling mechanism, that can prevent adhesion of a film of additives to a surface of an image bearing member and can form a high quality toner image even when a large quantity of external additives are added to the toner.
Another object of the present invention is to provide a novel image forming apparatus including a charging device that is applied with a voltage wherein an alternating current voltage is superimposed on a direct current voltage to charge an image bearing member, that can form a high quality toner image without adhesion of a film of additives to a surface of the image bearing member and can prevent wearing of the surface of the image bearing member and shortening of a useful life of the image bearing member even when a large quantity of external additives are added to the toner.
In order to achieve the above and other objectives, the present invention provides a novel image forming apparatus including an image bearing member on which an electrostatic latent image is formed, the image bearing member including a protective layer on a surface thereof, a developing device configured to develop the electrostatic latent image with toner to which an external additive is added to form a toner image, and a toner replenishing device configured to contain the toner and to supply air to the toner to replenish the developing device with the toner.
According to one aspect of the present invention, an image forming apparatus includes an image bearing member on which an electrostatic latent image is formed, the image bearing member including a protective layer on a surface thereof, a developing device configured to develop the electrostatic latent image with toner to form a toner image, a transfer device configured to transfer the toner image to a transfer material, a cleaning device configured to remove a toner that remains on the image bearing member after transfer of the toner image, and a toner returning device configured to return the toner removed from the image bearing member by the cleaning device to the developing device.
According to another aspect of the present invention, an image forming apparatus includes an image bearing member including a protective layer on a surface thereof, a charging device configured to be applied with a voltage wherein an alternating current voltage is superimposed on a direct current voltage to charge the image bearing member, an exposing device configured to expose a charged surface of the image bearing member to form an electrostatic latent image on the image bearing member, and a developing device configured to develop the electrostatic latent image with toner to which one part by weight or greater of an external additive is added to form a toner image.
Other objects, features, and advantages of the present invention will become apparent from the following detailed description when read in conjunction with the accompanying drawings.